A wound is considered to be chronic if there is little or no sign for healing after an appropriate therapy for 6-8 weeks. A chronic wound is usually not caused by external force, as seen in an abrasion or a scald, but has latent factors to delay or obstruct its normal healing process. Common types of chronic wounds include a pressure ulcer, a decubitus ulcer, a leg ulcer, and a diabetic foot ulcer.
Poor wound healing partially results from a causal link between local infection and poor tissue blood perfusion. As the tissue has an oxygen content less than 20 mm-Hg, the wound healing process stops owing to the fact that this hypoxia makes proliferation of fibroblast cells, collagen production, and capillary angiogenesis retarded. Such hypoxia also inhibits anti-bacterial activity of neutrophils, and therefore aerobic bacteria breed quickly. At the same time, this hypoxia condition further leads to rapid growth of anaerobic bacteria. That is, the rapid growth of aerobic bacteria and anaerobic bacteria can make the wound worse.